There is another way, with cactuar king. More automatic, but a little slower. Capture all the fiends in the thunder plains to unlock him. Using the ap weapon described above, set all overdrives to comrad. Get an auto phoenix ability on two of your characters, and enter the battle. Just keep pressing /\, until the cactuar king runs away. Buy more phoenixs, Battle again, and repeat.

There is another way, that requires more setup but makes you gain AP at insane rates. I'll put a link since it's too long to describe fully here, but, as a quick summary, it goes like this:

You need to unlock Don Tonberry at the Monster Arena. You get three weapons with the Overdrive -> AP ability (Triple Overdrive and Triple AP help, but they're not required). You'll need a character with a high number of kills, who will be in Stoic Overdrive mode and other two who will be in Comrade. You make the high-kill character attack Don Tonberry and it will counter with Karma, which does damage based on the amount of fiends the character has killed. This can reach very high damage, which will be converted into overdrive and then into AP. When Don Tonberry stops countering with Karma, flee. You will get tons of Sphere Levels.